FRANKFURT–Deutsche Börse AG is considering lowering the approval threshold for its proposed EUR25 billion ($27.63 billion) merger with London Stock Exchange Group PLC, according to people familiar with the matter.

That unexpected move would be the strongest sign yet of how committed both companies are to their tie-up despite mounting questions following the U.K.’s vote to leave the European Union.

The German exchange group is concerned that the tender offer that has been launched for its shares as part of the deal could fail to secure more than 75% of its stock by the Tuesday deadline, according to the people. Deutsche Börse is therefore considering lowering the minimum threshold to around 60%, they said.

Some large Deutsche Börse shareholders privately said they understood the strategic rationale of the combination but were hesitant to tender because they were uncertain about how Brexit may affect the transaction. As of Friday, around 25% of the shares had been tendered.

The issue is pressing because the deal would fall apart should sufficient shares not be tendered.

A final decision on whether to lower the minimum threshold will be made Monday morning, the people close to the deal said.

Write to Eyk Henning at eyk.henning@wsj.com

Breaking the story

Eyk Henning was first to report that Deutsche Börse AG was considering lowering the approval threshold to 60% from 75% to secure its proposed EUR25 billion ($27.63 billion) merger with London Stock Exchange Group PLC, according to sources. Deutsche Boerse formally announced it lowered the threshold via a press release Monday morning.

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